The present invention relates generally to nuclear reactors, and more particularly is directed to an apparatus used with a fuel assembly for controlling the nuclear reactivity by varying the volume of the moderator/coolant associated with the fuel rods of the assembly and, at the same time, adding a burnable poision gas in thereby improving the fuel utilization, thus allowing for lower fuel enrichments.
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. Conventional designs of these fuel assemblies include a plurality of fuel rods and hollow tubes or guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the guide thimbles. The guide thimbles are structural members which also provide channels for neutron absorber rods, burnable poison rods or neutron source assemblies which are all vehicles for controlling the reactivity of the reactor. Top and bottom nozzles on opposite ends thereof are secured to the guide thimbles in thereby forming an integral fuel assembly. Generally, in most reactors, a moderator/coolant such as water, is directed upwardly through aperatures in the lower core support plate and along the various fuel assemblies to receive the thermal energy therefrom. An example of such a fuel assembly structure can be seen in U.S. Pat. No. 4,326,419; granted to Donald J. Hill.
Since the nuclear industry's inception, core component design improvements have evolved in response to changes in regulatory requirements, manufacturing considerations, and power generation costs. Increasingly, utilities and fuel suppliers have focused ever more strongly on neutron economy and reduced power generation costs. These effects have been motivated by increased fuel and fuel enrichment costs. In response to these demands, designers have been working hard in developing new designs and in modifying existing designs to improve fuel utilization, as well as, in increasing safety margins in reactors.
It is known that improved fuel economy can be achieved in a PWR (Pressurized Water Reactor) by initially operating with a reduced H/U (hydrogen/uranium) ratio and then returning the ratio to normal somewhat later in the core cycle. The initial H/U reduction has the effect of increasing the epithermal part of the neutron spectrum at the expense of the thermal part. This results in increased breeding and decreased fission and fuel depletion rates. Since reactor fuel starts off with excess reactivity, this spectral shift represents no problem early in the core life; however, if the decrease in H/U were maintained through the entire core cycle, nothing would be gained because the higher fertile material absorption and lower fission rate would more than balance the gains from the increased breeding and lower burnup. Consequently, in order to properly take advantage of the increased breeding and lower burnup, it is necessary to return the H/U ratio back to its normal value. The net result would allow a reactor to be operated for a full core cycle with a reduced initial uranium enrichment. One of the ways of altering the H/U ratio which has been investigated involves the use of displacer rods. As the name implies, these rods are placed in the core to initially displace some of the moderating water and decrease the H/U ratio, and then, at some point during the core cycle, the displacement associated with these rods would be removed.
One approach considered for removing this displacement is through the use of movable mechanisms, similar to those associated with control rods. Such an approach is described in the above cross-referenced copending application of Trevor A. Francis, entitled "An Improved Water Displacer Rod Spider Assembly For A Nuclear Reactor Fuel Assembly". Among other unfavorable conditions, movable control mechanisms are expensive.
Another approach contemplated for removing the displacement is to have membranes provided on the ends of the displacer rods which are penetrated at some point in time to allow the rods to be filled with water. The basic idea makes use of a small heating element surrounding a specially indented end cap on the hollow displacer rod. At an appropriate time, the heater is turned on and the indented part of the end cap is weakened to the point where the external water pressure opens the end cap and fills the rod with water. The basic idea was expanded to include a manifold for each fuel assembly which would be constructed very similar to the spider-like control rod clusters presently used in reactors. All the rods in the cluster would be controlled by a single end-cap in the cluster head. The end-cap on each cluster would have an external plug connected to the heater inside. The procedure for changing the H/U ratio during a reactor cycle would be as follows: first, the reactor would be reduced to lower power or placed in a hot shut-down condition; the heaters in all the displacer rod clusters would be activated through heater power cables until all the end-caps have blown; and then, the reactor would be started up again. With the increased reactivity resulting from the higher H/U ratio, an elevated concentration of boron shim would have to be reintroduced into the primary coolant. Some of the problems anticipated with such an approach would be the reliability of the connectors and wiring when exposed to the pressure and corrosive capabilities of the reactor water, the potential failure of the rods themselves, what to do with the burst displacer rods after use since they are contaminated and thus inconvenient and impractical to transport and/or discard, and lastly, there is concern as to what would happen to such a displacer rod system in the case of LOCA (Loss of Coolant Activity) or other reactor problems.
The present inventors were aware of the teachings of the above described works and their shortcomings when they developed their alternative approach which is the subject of the present invention.